Pure Virtual Functions and Abstract Classes

Sometimes implementation of all function cannot be provided in a base class because we don’t know the implementation. Such a class is called abstract class. For example, let Shape be a base class. We cannot provide implementation of function draw() in Shape, but we know every derived class must have implementation of draw(). Similarly an Animal class doesn’t have implementation of move() (assuming that all animals move), but all animals must know how to move. We cannot create objects of abstract classes.

A pure virtual function (or abstract function) in C++ is a virtual function for which we don’t have implementation, we only declare it. A pure virtual function is declared by assigning 0 in declaration

#include<iostream> using namespace std;   class Base {    int x; public:     virtual void fun() = 0;     int getX() { return x; } };   class Derived: public Base {     int y; public:      void fun() { cout << "fun() called"; } };   int main(void) {     Derived d;     d.fun();     return 0; }

Output:

fun() called

Some Interesting Facts:


1) A class is abstract if it has at least one pure virtual function.

In the following example, Test is an abstract class because it has a pure virtual function show().





#include<iostream>

using namespace std;

 

class Test

{

   int x;

public:

    virtual void show() = 0;

    int getX() { return x; }

};

 

int main(void)

{

    Test t;

    return 0;

}






Output:
Compiler Error: cannot declare variable 't' to be of abstract
 type 'Test' because the following virtual functions are pure 
within 'Test': note:  virtual void Test::show() 


2) We can have pointers and references of abstract class type.
For example the following program works fine.





#include<iostream>

using namespace std;

 

class Base

{

public:

    virtual void show() = 0;

};

 

class Derived: public Base

{

public:

    void show() { cout << "In Derived \n"; }

};

 

int main(void)

{

    Base *bp = new Derived();

    bp->show();

    return 0;

}






Output:
In Derived 

3) If we do not override the pure virtual function in derived class, then derived class also becomes abstract class.
The following example demonstrates the same.





#include<iostream>

using namespace std;

class Base

{

public:

    virtual void show() = 0;

};

 

class Derived : public Base { };

 

int main(void)

{

  Derived d;

  return 0;

}





Compiler Error: cannot declare variable 'd' to be of abstract type 
'Derived'  because the following virtual functions are pure within
'Derived': virtual void Base::show()